1. Field of the Invention
The present invention relates to a liquid crystal display device having a projector type backlight, more specifically, to a shape of a refractive surface of a light guide plate composing the backlight.
2. Description of the related Art
A projector type backlight is conventionally used for a liquid crystal display device. FIG. 5 is a sectional view showing a backlight of a conventional liquid crystal display device. In this drawing, a backlight 100 is constructed to include a light source 102 for projecting light 101, and a light guide plate 103 for guiding the projected light 101 and diffusing it. A fluorescent tube, a cold cathode tube, or a hot cathode tube is used as the light source 102 shown in FIG. 5. The light source is located on a rear surface 104 side of the light guide plate 103 such that the projection direction of the light 101 is set perpendicular to the rear surface 104 of the light guide plate 103. Note that a width of the light 101 that enters the light guide plate 103 from the light source 102 at a right angle, that is, an incident light width 105 is dependent on a size of the light source 102.
The light guide plate 103 is made of a transparent plastic member and has a flat rectangular shape. The light guide plate 103 has at a side end portion a first refractive surface 106 for refracting the light 101 projected from the light source 102. The first refractive surface 106 is slanted at an angle of about 22 degrees with respect, to the plane of the light guide plate 103 and forms a slope covering the incident light width 105 of the light 101 as a whole. Note that a slant angle 108 of 22 degrees is slightly larger than a critical angle (about 19.8 degrees in the case of a polycarbonate member) at which light 107 reflected from the first refractive surface 106 is totally reflected from a second refractive surface 109 described later. In addition, the light guide plate 103 includes a reflective plate 110 on a rear side of the first refractive surface 106. The reflective plate 110 serves to reflect light (not shown) transmitted through the first refractive surface 106 without total reflection to return the light to the light guide plate 103.
In this light crystal display device, the light 101 which is projected from the light source 102 and has the constant incident light width 105 enters the light guide plate 103 from the rear surface 104 side thereof at a right angle and impinges on the first refractive surface 106 from an inner portion side of the light guide plate 103. One portion of the light 101 is reflected from the first refractive surface 106 and travels toward the second refractive surface 109 which is the rear surface 104 of the light guide plate 103. The other portion of the light 101 is transmitted through the first refractive surface 106, is reflected from the reflective plate 110 provided in the rear surface thereof, and enters the inner portion of the light guide plate 103 again to travel toward the second refractive surface 109. Thus, the light 107 is totally reflected from the second refractive surface 109 and then diffused into the light guide plate 103 while producing birefringence between a front surface 111 and the rear surface 104 of the light guide plate 103.
Here, as the number of refraction is increased, the light 101 transmitted through the light guide plate 103 is attenuated, reducing the intensity thereof. Thus, in order to increase a brightness of a display panel (not shown), it is preferable that the number of refraction of the light 101 is as small as possible. However, since the birefringence of the light 101 is produced in the inner portion of the light guide plate 103 in a conventional liquid crystal display device as described above, there is a problem in that a loss of light is caused. In order to prevent such a loss of light, there is also proposed a structure in which the slant angle 108 of the first refractive surface 106 is set to 45 degrees. Such a structure is shown in FIG. 6. As is apparent from FIG. 6, in the case of such an angle, the light 101 projected from the light source 102 is refracted at an angle at which the refraction direction is substantially parallel with the plane of the light guide plate 103 in the first refractive surface 106 and travels the inner portion of the light guide plate 103 in a straight line without birefringence. However, it is generally difficult to keep such a sufficient slant angle 108 while meeting the requirement for reduction in thickness of a display device (not shown). For example, as shown in FIG. 7, when a thickness of the light guide plate 103 is reduced while keeping the slant angle 108 of 45 degrees, since an area of the first refractive surface 106 in a width direction becomes small, the incident light width 105 of the light 101 from the light source 102 cannot be sufficiently covered. Thus, since a portion 112 of the incident light 101 is transmitted through the surface 111 of the light guide plate 103 without impinging on the first refractive surface 106, there still remains a problem in that a loss of light is caused.